1. Field of the Invention
The present invention relates generally to fuel assemblies for a nuclear reactor and, more particularly, is concerned with a boiling water reactor (BWR) fuel assembly having localized neutron absorber strips placed on its outer tubular channel for facilitating calibration of Local Power Range Monitor (LPRM) neutron flux detectors.
2. Description of the Prior Art
Typically, large amounts of energy are released through nuclear fission in a nuclear reactor with the energy being dissipated as heat in the elongated fuel elements or rods of the reactor. The heat is commonly removed by passing a coolant in heat exchange relation to the fuel rods so that the heat can be extracted from the coolant to perform useful work.
In nuclear reactors generally, a plurality of the fuel rods are grouped together to form a fuel assembly. A number of such fuel assemblies are typically arranged in a matrix to form a nuclear reactor core capable of a self-sustained, nuclear fission reaction. The core is submersed in a flowing liquid, such as light water, that serves as the coolant for removing heat from the fuel rods and as a neutron moderator. Specifically, in a BWR the fuel assemblies are typically grouped in clusters of four with one control rod associated with each four assemblies. The control rod is insertable between the fuel assemblies for controlling the reactivity of the core. Each such cluster of four fuel assemblies surrounding a control rod is commonly referred to as a fuel cell of the reactor core.
A typical BWR fuel assembly in the cluster is ordinarily formed by a N by N array of the elongated fuel rods. The bundle of fuel rods are supported in laterally spaced-apart relation and encircled by an outer tubular channel having a generally rectangular cross-section. Examples of such fuel assemblies are illustrated and described in U.S. Pat. Nos. (3,349,004) to Lass et al, (3,689,358) Smith et al, (3,802,995) Fritz et al, (4,560,532) Barry et al and (4,649,021) Taleyarkhan and in a Canadian Pat. No. (1,150,423) to Anderson et al.
A BWR core typically includes several LPRM strings dispersed throughout the core. These strings are located inbetween the corner locations of four fuel assemblies. Each string includes a hollow tube with four neutron detectors located at discrete axial locations. During reactor operation these detectors provide crucial local power monitoring information. However, the detectors need to be calibrated at specific time intervals with a movable tip probe that is inserted from the bottom of the core, into selected detector string tubes. This calibration is necessary for maintaining the accuracy/fidelity of the LPRM readings on the control console.
In a BWR core made up of General Electric (GE-8.times.8) fuel assemblies as the tip probe is inserted in the string tube, its relative position is evaluated from the location of neutron flux dips caused by Inconel fuel rod spacers located axially along the fuel assembly. The Inconel spacers, usually seven in number, act as neutron absorbers and hence such dips occur.
In reload situations where Westinghouse Electric (W-QUAD+) BWR fuel assemblies are used to replace selected General Electric BWR fuel assemblies, it is highly likely that Westinghouse fuel assemblies will end up replacing GE fuel assemblies at one of the LPRM string locations. The Westinghouse fuel assembly, designed for optimized fuel cycle cost benefits, employs an all-Zircaloy spacer design. However, these Westinghouse Zircaloy spacers will not produce local neutron flux dips like the GE Inconel spacers. Furthermore, the six Zircaloy spacers in the adjacent fuel rod subassembly of the Westinghouse fuel assembly are located at axial positions different from the axial positions of the seven Inconel spacers in the GE fuel assembly. Hence, proper positioning of the tip probe for calibration purposes becomes impossible with current plant setup. Such a situation could lead to NRC-imposed uncertainty penalties in the form of plant derates.
Consequently, a need exists for an effective means of providing an indicator for locating the LPRM detectors in plants where GE BWR fuel assemblies are replaced with Westinghouse BWR fuel assemblies.